Well 1st of all, electronics works better at lower temperature (it has something to do with conductivity i think). But I don't think it will affect raw performance of your GPU to lower the temp. Did you monitored your GPU temps under load or at idle state? 70C at idle is kinda high, but 70C at load is more than acceptable.
For the open case question, I would not recommend removing the entire pannel since the air flow created by your case fans will be messed up by that giant opening. You're better off with small openings feeding your case of fresh air and following a path from bottom front to top back of the case. If the side pannel is open, you wont benefit from the airflow generated by your case fans.
Hope this help!